Melanins serve as the primary skin coloration pigments. In human skin, two primary melanin types are present. The black/brown melanin, eumelanin, provides primary skin coloration with eumelanin more prevalent in humans with dark skin color. Eumelanin is derived from hydroxy-substituted aromatic amino acids such as L-tyrosine and L-DOPA. Eumelanin is believed to include 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid polymers. The yellow to red pheomelanins are found in humans of both dark and light skin coloration with the greatest quantities found in red hair. Pheomelanins are formed from sulfur-containing molecules and differ from eumelanin in the presence of benzothiazine and benzothiazole units in the oligomers when the amino acid L-cysteine is present.
Melanin is produced in melanocytes found at the dermis-epidermis junction. The melanin is transferred from the melanocytes to keratinocytes that form at this junction and then migrate to the outer epidermal layer transporting the melanin to the skin surface. The resulting coloration of the skin is dependent on the rate of transfer of melanin to keratinocytes, as well as the number, size, and melanin content of the keratinocytes in particular regions of the skin.
Irregular skin pigmentation occurs from the uneven distribution of melanocytes. Hyperpigmentation, or the presence of focused or unfocused skin pigmentation of darker appearance than the surrounding tissue is common. Hyperpigmentation may be the result of pregnancy where women experience a darkening of some regions of the skin often referred to as melasma or the mask of pregnancy. Melasma may also be linked to the use of hormone based birth control or hormone replacement therapy. Many people develop dark spots with age due to sun exposure known as solar lentigenes or liver spots. Other forms of hyperpigmentation may occur as a result of persistent acne, bums, bites or other skin injuries.
It is commonly desirable to reduce or remove these and other types of skin pigmentations. People that present with excessive or unwanted age spots or freckles may wish to have these areas less pronounced relative to surrounding skin tissue. Several compositions have been developed to target such areas. It is all too common, however, for these compositions to either fail to provide the desired effects, or to produce unwanted irritation of the skin that can actually stimulate melanocyte activity paradoxically darkening the skin. Thus, there remains a need for improved compositions and methods for altering skin pigmentation.